The Mercedes S-class cabriolet is the best car you can buy this side of a Bentley GTC

Mercedes has a bit of a problem. It’s a brand that makes a lot of very clever things – infra-red screens so you can see in the dark, otherly ways of detecting pedestrians that stumble into the road, cars that brake for you, steer for you, go very fast, use very little fuel, the list goes on. But it’s also a brand that makes a lot of cars to bolt them to. As we type, you can buy an A, B, C, CLA, CLS, E, G, GLA, GLC, GLE, GLS, S, SL, SLC, V-class Mercedes. Oh, and an AMG GT.

This means that your compact crossover can now have the power of Thor and technology that literally competes with sentience. Which leaves the S-class in a difficult position. It was a car that was once on the prow of innovation, but the company’s wealth of clever and lovely parts isn’t reserved for the flagship anymore, so it’s drifted out of the spotlight and become more of an expensive Uber than a peerless pluto-thing you actually want to own.

But now there’s this; Mercedes’ answer to the S-Class desirability predicament. It’s the first rag-top S since the 1965 W111 model (you know the one. It’s in The Hangover) and with its rooflessness, scale, largesse and enticing haberdashery it’s a robust bid to make the S brand chic again.

As per protocol, there is a lot of power. We’ll cut to the chase, because cutting to the chase is what this car does best - even though we’re in the mid-range S63 model, there’s a twin-turbo 5.5-litre V8 bolted to a four-wheel drive system that gets it from 0-62mph in 4.2 seconds. That’s nearly supercar-fast, but in a 2110kg, five-by-two metre convertible it’s like driving a leather and carbon fibre mud slide. The power is big, everywhere in the rev range and vast enough to rule out the even more muscular S65 V12 version (pictured) from serious consideration. Mercifully, it’ll burble along at civilised speeds as well.

So it’s got speed covered, but what about the interior? There’s a lot of soft, shimmering stuff that manages to look expensive and crafted without ever being gauche – something Mercedes has always been good at. OK, so the seats still look a bit like the ones fitted to anything north of the A-class, and by extension those pay-per-minute massaging chairs nobody ever uses at the airport, but they’re that sweet spot between Roll-Royce soft and Caterham hard. They can also heat, cool, and, indeed, massage you in lots of exciting ways - actually, in the S-class, Mercedes takes your body temperature very seriously, warming you up in four ways, including heated armrests and heated headrests.

As you’d expect, the tech extends quite a lot further than the sticky leather. But there’s not a lot here that’s exclusive to the S. The thermal-imaging camera’s available on the A-class, as are other headline features like lane departure correction, radar cruise control and digital dashboards, albeit smaller. A scan through the brochures tells us that Swarovski crystals in the headlights (no, really), scented air-con (again, really) and roof are more or less it.

But the roof is worth talking about. It’s the largest fabric lid applied to a production car and can be raised and lowered in 20 seconds flat when you’re going up to and including 37mph. That’s not the best bit, though – the only thing that comes close in terms of noise isolation is a Rolls-Royce Dawn, and that’s a good £100,000 more expensive.

Unfortunately, a clever top’s not quite enough to make the S… exciting. Especially considering that it looks just like a scaled up version of the C-class cabriolet and therefore everything else Mercedes builds. That’s not to say it isn’t apocalyptically fast, a technical tour de force and – on paper - the best convertible you can buy this side of a Bentley GTC. But you’ll struggle to justify this car on anything other than rational grounds, and rationality doesn’t really come in to buying a £150,000 convertible, which makes this a lot of very impressive things in search of a USP.